125-year-old Indian seismic mystery solved

More than a century after the 8.0-magnitude earthquake in India, researchers have determined the epicenter of the massive earthquake.

By Davitia James, Temblor Earthquake News Extern (@davitiaa)

Quote: James D. , 2021, 125-year-old Indian seismic mystery solved, Temblor, http://doi.org/10.32858/temblor.223

On June 12, 1897, an earthquake measuring 8 on the Richter scale struck most of the Indian subcontinent. Since then, several sites have been proposed as the origin of the earthquake. Now, using original records of the event and 3D models of the earthquake’s behavior, researchers have determined the epicenter, according to a new study published in The Seismic Record. Their work places the starting point at the intersection of the Chidrange and Oldham faults in the northwest region of the Shillong Plateau – a broad, flat region of Himalayan territory in northeastern India.

Map from British geologist Richard Dixon Oldham’s report on the 1897 earthquake. Dark brown shows the proposed epicenter range; Areas that reported severe damage are shaded in medium brown; The areas where the earthquake was felt are covered in the lightest shade. Credit: Oldham, R. , 1899, “Report on the Great Earthquake of June 12, 1897.”

on top of the world

The Himalayas are the result of the collision of the Indian and Eurasian tectonic plates. Earth’s highest mountains above sea level are found in this area – Mount Everest is the most famous – and the area has been where the major earthquakes throughout recorded history have been. The 1897 earthquake, known as the Assam earthquake, caused the plateau to rise at least 33 feet (10 meters) in the north and resulted in extensive loss of life and structural damage. Reports indicate that more than 1,500 people died and that the damage was extensive over an area roughly the size of England (Oldham, 1899).

Map of 19th century earthquakes in the Himalayas. Credit: Bilham, R., 2019, “Himalayan earthquakes: a review of historical earthquakes and slip potential in the early twenty-first century.”

The big debate

Experts involved in the Assam earthquake generally accept that the magnitude of the moment – a description of the amount of energy released by the earthquake – lies between -8.0 and -8.4. However, the location of the epicenter has not been agreed upon.

In 1899, British geologist Richard Dickson Oldham, a scientist with the Geological Society of India, wrote an extensive report on this earthquake and first noted that different seismic waves arrive at different speeds. Using damage reports, felt reports and ground acceleration reports, along with field observations and very early seismometers from stations around the world, Oldham estimated that the epicenter was in the Shillong Plateau. He drew a map with a roughly triangular area from which he assumed the rupture began.

His report and others over the past 124 years have focused on diving faults to the north in the Himalayas, but a 2001 study (Pelham and England, 2001) placed the epicenter on a 68-mile (110 km) fault now called the Oldham fault. The Oldham is a buried fault that lowers far south from the Himalayas.

Buried faults never reach the surface but they still pose significant threats. Across the world, scientists are investigating why these types of faults cause so many vibrations and how to assess their risks.

The idea of ​​the epicenter of this large earthquake occurring on a buried fault is controversial, says György Hetényi, co-author of the new study and professor at the University of Lausanne where the authors are based. Some experts tend to believe that most large earthquakes — greater than magnitude 8 — must occur on visible faults, Hetényi says.

Find the puzzle pieces

Hetényi and co-author Shiba Subedi decided to see if they could more accurately locate the epicenter of the 1897 earthquake, as well as when the event occurred.

Subedi and Hetney examined damage reports and direct observations of the earthquake—such as notes about when the shaking occurred at train stations—as well as seismic records kept from stations around the world. Although seismometers were invented only a few years before the 1897 Assam earthquake, a number of them have already been installed. Subedi and Hetney combed historical records to calculate the time of arrival of seismic waves. They then combined these arrival times with modern velocity models to detect how far and when the earthquake occurred.

Hetényi says using the arrival times and waveforms from international seismometers has been challenging. “Information regarding arrival times is not as accurate as it is today.” Hetényi and Subedi can use records whose access times are only seconds. In addition, Hitini says, not all stations were operating when the earthquake struck. Some, for example, had their registration paper replaced at the same moment for the earthquake, and thus had no record of the event. In the end, though, their search yielded P-wave and S-wave data from 32 different seismic stations.

liquefaction in a sandy pit in northeastern India. Credit: Giorgi Hetney, from the archives of the Oldham Earthquake Report.

solve the puzzle

Hetényi and Subedi determined that the earthquake’s start time was 11:06:46 UTC, plus or minus 15 seconds. With better constrained timing, and knowledge of how quickly different earthquake waves travel, they were able to calculate the location using a computer model that takes into account changes in the crust and mantle at different locations.

The resulting point was 26.0°N, 90.7°E. This places the epicenter “at the western end of the Oldham Fault, at its junction with the Chidrange Fault,” the team wrote. This site lies within the 1899 Oldham scheme and is further north than most estimates, but within 25 miles (40 km) of other proposals, including one proposed by Oldham. The site removes other faults, such as the main forward direction that drops northward below the Himalayas, from contention.

Many new puzzles to choose from

This is the first study to use original instrument recordings to improve epicenter estimates for the Assam earthquake. The study supports the idea that large earthquakes can occur on buried faults, says Roger Pelham, a professor emeritus at the University of Colorado Boulder who was not involved in the study.

Finding this ancient fault buried as the origin of a devastating earthquake is important for risk assessment in this part of the Himalayas. “Modern seismic hazard studies begin with fault maps on the surface,” Hetényi says. He adds that there are other faults buried in the area, so improving the seismic coverage here is an essential step to broadening the understanding of these subsurface faults. In turn, it indicates that this will hopefully improve preparedness for future earthquakes in this region and other seismically active regions located within plate tectonics.

Pelham says he looks forward to future research in the area, particularly around the nearby Chedrang Fault. The northern edge of the Shillong Plateau shows approximately 36 feet (11 m) of vertical displacement, and movement in the area has offset boulders crossing the Chidrange River, he notes. With faults extending across most of the crust and perhaps the largest displacement ever measured for a single earthquake, this region of visible and hidden faults offers endless opportunities for further seismic surveillance for decades to come, Pelham says.

references

Subedi, S.; and Hetényi, c. (2021). Precise determination of the location of the 1897 Great Shillong Plateau earthquake using regional seismic and seismic phase data, seismic record. 1 (3), 135-144, doi: 10.1785/0320210031.

in-depth reading

Pelham, R.; (2008). Tom La Touche and the Great Assam Earthquake of June 12, 1897: Letters from the Epicenter, Seismol. Precision. Lett. 79, no. 3, 426-437.

Chen, W.P., and P. Molnar (1990). Source parameters of earthquakes and internal deformation under the Shillong Plateau and the Northern Indoburman Ranges, J. Geoves. Precision. 95, no. B8, 12527-12552.

England, P., and R. Bilham (2015). The Shillong Plateau and the Great Assam Earthquake of 1897, Tectonic 34, no. 9, 1792-1812.

Gutenberg, b. (1956). The Great Earthquakes 1896–1903, EOS Trans. I be. Geophysics. Union 37, no. 5, 608-614.

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